diff --git a/README.md b/README.md
index bdc8dac..e62ffc3 100644
--- a/README.md
+++ b/README.md
@@ -141,7 +141,9 @@ The first line in this header file must be in the form:
 If the name is followed by the `!` modifier, then it is *unprefixable* (see
 the previous section for details). The name can also be followed by `:` and a
 list of base checks. Such checks are automatically inserted before the rest of
-the lines in the resulting substitution.
+the lines in the resulting substitution. One notable check that you may
+want to use as a base is [`LIBC_VERSION`][libc-version] (see comments for
+details).
 
 Subsequent lines should be C-style comments or preprocessor directives that
 `#define` or `#undef` `<NAME>` depending on whether the feature is available
@@ -178,3 +180,4 @@ ways that deal with duplication (for example, include guards).
 [module-in]: https://build2.org/build2/doc/build2-build-system-manual.xhtml#module-in
 [proj-config]: https://build2.org/build2/doc/build2-build-system-manual.xhtml#proj-config
 [checks]: https://github.com/build2/libbuild2-autoconf/tree/master/libbuild2-autoconf/libbuild2/autoconf/checks/
+[libc-version]: https://github.com/build2/libbuild2-autoconf/tree/master/libbuild2-autoconf/libbuild2/autoconf/checks/LIBC_VERSION.h
diff --git a/libbuild2-autoconf-tests/checks/byte-order/.gitignore b/libbuild2-autoconf-tests/checks/byte-order/.gitignore
index 534ba0c..9d45c9a 100644
--- a/libbuild2-autoconf-tests/checks/byte-order/.gitignore
+++ b/libbuild2-autoconf-tests/checks/byte-order/.gitignore
@@ -1,2 +1,3 @@
+config.h
 driver
 driver-posix
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h b/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h
index 475dc32..35ab553 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h
@@ -43,8 +43,8 @@
 #    define BIG_ENDIAN    4321
 #    define LITTLE_ENDIAN 1234
 #    define BYTE_ORDER    LITTLE_ENDIAN
-#  elif defined(__BYTE_ORDER__) &&              \
-        defined(__ORDER_BIG_ENDIAN__) &&        \
+#  elif defined(__BYTE_ORDER__) &&       \
+        defined(__ORDER_BIG_ENDIAN__) && \
         defined(__ORDER_LITTLE_ENDIAN__)
      /* GCC, Clang (and others, potentially).
       */
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_CLOCK_GETTIME.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_CLOCK_GETTIME.h
new file mode 100644
index 0000000..4c9302d
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_CLOCK_GETTIME.h
@@ -0,0 +1,14 @@
+// HAVE_CLOCK_GETTIME : LIBC_VERSION
+
+#undef HAVE_CLOCK_GETTIME
+
+/* Since Linux 2.6/glibc 2.2 (based on glibc commit history), FreeBSD 3.0,
+ * OpenBSD 2.1, NetBSD 1.4, and Mac OSX 10.12.
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 2)     || \
+    BUILD2_AUTOCONF_FREEBSD_PREREQ(3, 0)   || \
+    BUILD2_AUTOCONF_OPENBSD_PREREQ(199706) || \
+    BUILD2_AUTOCONF_NETBSD_PREREQ(1, 4)    || \
+    BUILD2_AUTOCONF_MACOS_PREREQ(10, 12)
+#  define HAVE_CLOCK_GETTIME 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_DLOPEN.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_DLOPEN.h
new file mode 100644
index 0000000..87ff5b6
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_DLOPEN.h
@@ -0,0 +1,13 @@
+// HAVE_DLOPEN : LIBC_VERSION
+
+#undef HAVE_DLOPEN
+
+/* Since Linux/glibc 2.0 and all versions of other supported UNIXes.
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 0) || \
+    defined(__FreeBSD__)               || \
+    defined(__OpenBSD__)               || \
+    defined(__NetBSD__)                || \
+    (defined(__APPLE__) && defined(__MACH__))
+#  define HAVE_DLOPEN 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EVENTFD.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EVENTFD.h
new file mode 100644
index 0000000..ebc2c88
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EVENTFD.h
@@ -0,0 +1,13 @@
+// HAVE_EVENTFD : LIBC_VERSION
+
+#undef HAVE_EVENTFD
+
+/* Since Linux/glibc 2.8, FreeBSD 13.
+ *
+ * @@ TODO eventfd() looks likely to be added to NetBSD 10 which does not yet
+ *    have a release date.
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 8) || \
+    BUILD2_AUTOCONF_FREEBSD_PREREQ(13, 0)
+#  define HAVE_EVENTFD 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EXPLICIT_BZERO.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EXPLICIT_BZERO.h
index 8f360ae..04b898d 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EXPLICIT_BZERO.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EXPLICIT_BZERO.h
@@ -1,12 +1,11 @@
-// HAVE_EXPLICIT_BZERO
+// HAVE_EXPLICIT_BZERO : LIBC_VERSION
 
 #undef HAVE_EXPLICIT_BZERO
 
 /* Since FreeBSD 11, OpenBSD 5.5, and glibc 2.25.
  */
-#if (defined(__FreeBSD__) && __FreeBSD__ >= 11) ||                      \
-    (defined(__OpenBSD__) && defined(OpenBSD) && OpenBSD >= 201405) ||  \
-    (defined(__GLIBC__) && defined(__GLIBC_MINOR__) &&                  \
-     (__GLIBC__ > 2 || __GLIBC__ == 2 && __GLIBC_MINOR__ >= 25))
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 25)   || \
+    BUILD2_AUTOCONF_FREEBSD_PREREQ(11, 0) || \
+    BUILD2_AUTOCONF_OPENBSD_PREREQ(201405)
 #  define HAVE_EXPLICIT_BZERO 1
 #endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EXPLICIT_MEMSET.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EXPLICIT_MEMSET.h
index 424238e..7d22f0d 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EXPLICIT_MEMSET.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_EXPLICIT_MEMSET.h
@@ -1,9 +1,9 @@
-// HAVE_EXPLICIT_MEMSET
+// HAVE_EXPLICIT_MEMSET : LIBC_VERSION
 
 #undef HAVE_EXPLICIT_MEMSET
 
 /* Since NetBSD 7.0.0.
  */
-#if defined(__NetBSD_Version__) && __NetBSD_Version__ >= 700000000
+#if BUILD2_AUTOCONF_NETBSD_PREREQ(7, 0)
 #  define HAVE_EXPLICIT_MEMSET 1
 #endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FUTIMENS.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FUTIMENS.h
new file mode 100644
index 0000000..63b8a29
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FUTIMENS.h
@@ -0,0 +1,14 @@
+// HAVE_FUTIMENS : LIBC_VERSION
+
+#undef HAVE_FUTIMENS
+
+/* Since Linux/glibc 2.6, FreeBSD 10.3, OpenBSD 5.0, NetBSD 6.0, Mac OS 10.6
+ * (2009).
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 6)     || \
+    BUILD2_AUTOCONF_FREEBSD_PREREQ(10, 3)  || \
+    BUILD2_AUTOCONF_OPENBSD_PREREQ(201111) || \
+    BUILD2_AUTOCONF_NETBSD_PREREQ(6, 0)    || \
+    BUILD2_AUTOCONF_MACOS_PREREQ(10, 6)
+#  define HAVE_FUTIMENS 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FUTIMES.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FUTIMES.h
new file mode 100644
index 0000000..a526d0c
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FUTIMES.h
@@ -0,0 +1,15 @@
+// HAVE_FUTIMES : LIBC_VERSION
+
+#undef HAVE_FUTIMES
+
+/* Since Linux/glibc 2.3, FreeBSD 3.x, OpenBSD 1.2 (but the `OpenBSD` macro
+ * was only added in 2.0 with a value of 199610), NetBSD 1.2, Mac OS (all
+ * versions).
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 3)     || \
+    BUILD2_AUTOCONF_FREEBSD_PREREQ(3, 0)   || \
+    BUILD2_AUTOCONF_OPENBSD_PREREQ(199610) || \
+    BUILD2_AUTOCONF_NETBSD_PREREQ(1, 2)    || \
+    (defined(__APPLE__) && defined(__MACH__))
+#  define HAVE_FUTIMES 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_GETAUXVAL.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_GETAUXVAL.h
new file mode 100644
index 0000000..2fdefe9
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_GETAUXVAL.h
@@ -0,0 +1,9 @@
+// HAVE_GETAUXVAL : LIBC_VERSION
+
+#undef HAVE_GETAUXVAL
+
+/* Since glibc 2.16.
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 16)
+#  define HAVE_GETAUXVAL 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_GETENTROPY.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_GETENTROPY.h
new file mode 100644
index 0000000..0404413
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_GETENTROPY.h
@@ -0,0 +1,15 @@
+// HAVE_GETENTROPY : LIBC_VERSION
+
+#undef HAVE_GETENTROPY
+
+/* Since Linux/glibc 2.25, OpenBSD 5.6, FreeBSD 12, and Mac OS 12.
+ *
+ * @@ TODO Update when NetBSD releases its support (see
+ *    https://mail-index.netbsd.org/tech-userlevel/2020/05/02/msg012333.html).
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 25)    || \
+    BUILD2_AUTOCONF_FREEBSD_PREREQ(12, 0)  || \
+    BUILD2_AUTOCONF_OPENBSD_PREREQ(201411) || \
+    BUILD2_AUTOCONF_MACOS_PREREQ(10, 12)
+#  define HAVE_GETENTROPY 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_INOTIFY.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_INOTIFY.h
new file mode 100644
index 0000000..5e23c4b
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_INOTIFY.h
@@ -0,0 +1,9 @@
+// HAVE_INOTIFY : LIBC_VERSION
+
+#undef HAVE_INOTIFY
+
+/* Since Linux 2.6.3/glibc 2.4.
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 4)
+#  define HAVE_INOTIFY 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_LINKAT.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_LINKAT.h
new file mode 100644
index 0000000..55bd253
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_LINKAT.h
@@ -0,0 +1,15 @@
+// HAVE_LINKAT : LIBC_VERSION
+
+#undef HAVE_LINKAT
+
+/* Since Linux 2.6.16/glibc 2.4, FreeBSD 8.0, OpenBSD 5.0, NetBSD 7.0 (it was
+ * only partially implemented in earlier versions according to the manpage),
+ * Mac OS (all versions).
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 4)     || \
+    BUILD2_AUTOCONF_FREEBSD_PREREQ(8, 0)   || \
+    BUILD2_AUTOCONF_OPENBSD_PREREQ(201111) || \
+    BUILD2_AUTOCONF_NETBSD_PREREQ(7, 0)    || \
+    (defined(__APPLE__) && defined(__MACH__))
+#  define HAVE_LINKAT 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POLL.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POLL.h
new file mode 100644
index 0000000..cc03b70
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POLL.h
@@ -0,0 +1,15 @@
+// HAVE_POLL : LIBC_VERSION
+
+#undef HAVE_POLL
+
+/* Since Linux 2.1.23/glibc (all versions; emulated using select() on older
+ * kernels), FreeBSD (all versions), OpenBSD (all versions), NetBSD 1.3, Mac
+ * OS (all versions).
+ */
+#if defined(__GLIBC__)                  || \
+    defined(__FreeBSD__)                || \
+    defined(__OpenBSD__)                || \
+    BUILD2_AUTOCONF_NETBSD_PREREQ(1, 3) || \
+    (defined(__APPLE__) && defined(__MACH__))
+#  define HAVE_POLL 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POLLTS.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POLLTS.h
new file mode 100644
index 0000000..518ef2b
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POLLTS.h
@@ -0,0 +1,9 @@
+// HAVE_POLLTS : LIBC_VERSION
+
+#undef HAVE_POLLTS
+
+/* Since NetBSD 3.0.
+ */
+#if BUILD2_AUTOCONF_NETBSD_PREREQ(3, 0)
+#  define HAVE_POLLTS 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POSIX_FALLOCATE.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POSIX_FALLOCATE.h
new file mode 100644
index 0000000..8cfb2ab
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POSIX_FALLOCATE.h
@@ -0,0 +1,14 @@
+// HAVE_POSIX_FALLOCATE : LIBC_VERSION
+
+#undef HAVE_POSIX_FALLOCATE
+
+/* Since Linux/glibc 2.2, FreeBSD 9.0
+ *
+ * posix_fallocate(3) on Linux says since glibc 2.1.94 but use 2.2 as
+ * https://sourceware.org/glibc/wiki/Glibc%20Timeline does not show that
+ * version and the glibc macros can't check for patch versions.
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 2) || \
+    BUILD2_AUTOCONF_FREEBSD_PREREQ(9, 0)
+#  define HAVE_POSIX_FALLOCATE 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_PPOLL.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_PPOLL.h
new file mode 100644
index 0000000..fe0d394
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_PPOLL.h
@@ -0,0 +1,11 @@
+// HAVE_PPOLL : LIBC_VERSION
+
+#undef HAVE_PPOLL
+
+/* Since Linux 2.6.16/glibc 2.4, FreeBSD 11, OpenBSD 5.4.
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 4)    || \
+    BUILD2_AUTOCONF_FREEBSD_PREREQ(11, 0) || \
+    BUILD2_AUTOCONF_OPENBSD_PREREQ(201311)
+#  define HAVE_PPOLL 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_RENAMEAT2.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_RENAMEAT2.h
new file mode 100644
index 0000000..3d26765
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_RENAMEAT2.h
@@ -0,0 +1,9 @@
+// HAVE_RENAMEAT2 : LIBC_VERSION
+
+#undef HAVE_RENAMEAT2
+
+// Since Linux 3.15/glibc 2.28.
+//
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 28)
+#  define HAVE_RENAMEAT2 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STATX.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STATX.h
new file mode 100644
index 0000000..385a48b
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STATX.h
@@ -0,0 +1,9 @@
+// HAVE_STATX : LIBC_VERSION
+
+#undef HAVE_STATX
+
+/* Since Linux 4.11/glibc 2.28.
+ */
+#if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 28)
+#  define HAVE_STATX 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/LIBC_VERSION.h b/libbuild2-autoconf/libbuild2/autoconf/checks/LIBC_VERSION.h
new file mode 100644
index 0000000..1057762
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/LIBC_VERSION.h
@@ -0,0 +1,164 @@
+// LIBC_VERSION!
+
+/* Include headers required to make various libc version number macros
+ * available.
+ *
+ * Note that besides the platform-specific macros listed below, this
+ * check also defines the following helper macros:
+ *
+ * BUILD2_AUTOCONF_GLIBC_PREREQ(maj, min)
+ * BUILD2_AUTOCONF_FREEBSD_PREREQ(maj, min)
+ * BUILD2_AUTOCONF_OPENBSD_PREREQ(yyyymm)
+ * BUILD2_AUTOCONF_NETBSD_PREREQ(maj, min)
+ * BUILD2_AUTOCONF_MACOS_PREREQ(maj, min)
+ *
+ * Note that all of the above macros are always defined and evaluate to false
+ * on platforms to which they do not apply. Which means that instead of:
+ *
+ * #if defined(__GLIBC__) && BUILD2_AUTOCONF_GLIBC_PREREQ(2, 3)
+ *
+ * You can just write:
+ *
+ * #if BUILD2_AUTOCONF_GLIBC_PREREQ(2, 3)
+ *
+ * Note also that for checking just for the specific platform we still use one
+ * of the system macros below. For example:
+ *
+ * #if defined(__GLIBC__)
+ * #if defined(__FreeBSD__)
+ * #if defined(__OpenBSD__)
+ * #if defined(__NetBSD__)
+ * #if defined(__APPLE__) && defined(__MACH__)
+ *
+ * Note that just __APPLE__ covers all the Apple platforms (MacOS, iOS, etc).
+ *
+ * Macros for detecting platforms and their versions:
+ *
+ * __GLIBC__: The glibc major version number.
+ *
+ * __GLIBC_MINOR__: The glibc minor version number.
+ *
+ * __GLIBC_PREREQ(major,minor): Return 1 if a given version number is greater
+ *                              than or equal to the current glibc version
+ *                              (__GLIBC__.__GLIBC_MINOR__), or 0 otherwise.
+ *
+ * __FreeBSD__: Defined if on FreeBSD. The value is the FreeBSD major version
+ *              number but it's typically used only for OS detection. This is
+ *              a predefined macro so no header needs to be included.
+ *
+ * __FreeBSD_version: The FreeBSD version in the following format: MMmmXXX
+ *                    where `MM` is the major and `mm` the minor version
+ *                    numbers. The `XXX` portion can be disregarded for the
+ *                    purposes of feature checking (because it's too
+ *                    fine-grained).
+ *
+ * __OpenBSD__: Defined if on OpenBSD. This is a predefined macro so no header
+ *              needs to be included.
+ *
+ * OpenBSD: The OpenBSD version in the following format: YYYYMM (year and
+ *          month of release). There are always two releases per year: one
+ *          with an odd-numbered minor version earlier in the year and one
+ *          with an even-numbered minor version later in the year. OpenBSD 7.0
+ *          was released in October 2021 so its OpenBSD macro value is 202110.
+ *
+ *
+ * __NetBSD__: Defined if on NetBSD. This is a predefined macro so no header
+ *             needs to be included.
+ *
+ * __NetBSD_Version__: The NetBSD version in the following format: MMmmrrpp00
+ *                     where `MM` is the major and `mm` the minor version
+ *                     numbers, `rr` is always 0 after NetBSD version 2.0I,
+ *                     and `pp` is the patch level. The `rrpp00` portion can
+ *                     be disregarded for the purposes of feature checking
+ *                     (because it's too fine-grained).
+ *
+ * __MAC_OS_X_VERSION_MIN_REQUIRED: The minimum supported Mac OS version. Up
+ *                                  to Mac OS X 10.9, formatted as `MMm0`
+ *                                  where `m` was the (always single-digit)
+ *                                  minor version. Since Mac OS X 10.10 it is
+ *                                  formatted as `MMmmpp` where `mm` is the
+ *                                  (now potentially double-digit) minor
+ *                                  version and `pp` is the patch version.
+ */
+#if defined(__linux__)
+#  include <features.h> /* __GLIBC__, __GLIBC_MINOR__, __GLIBC_PREREQ() */
+#elif defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__)
+#  include <sys/param.h> /* __FreeBSD_version, OpenBSD, __NetBSD_Version__ */
+#elif defined(__APPLE__)
+#  include <Availability.h> /* __MAC_OS_X_VERSION_MIN_REQUIRED */
+#endif
+
+/* BUILD2_AUTOCONF_GLIBC_PREREQ(maj, min)
+ *
+ * Return 1 if the given version number is >= the glibc version, or 0
+ * otherwise.
+ *
+ * __GLIBC_PREREQ() was only added in glibc 2.2 (released in 2000) so define
+ * BUILD2_AUTOCONF_GLIBC_PREREQ() using its latest glibc implementation if it
+ * is not defined.
+ */
+#if defined(__GLIBC__) && defined(__GLIBC_MINOR__)
+#  if defined(__GLIBC_PREREQ)
+#    define BUILD2_AUTOCONF_GLIBC_PREREQ(maj, min) __GLIBC_PREREQ(maj, min)
+#  else
+#    define BUILD2_AUTOCONF_GLIBC_PREREQ(maj, min) \
+       ((__GLIBC__ << 16) + __GLIBC_MINOR__ >= ((maj) << 16) + (min))
+#  endif
+#else
+#  define BUILD2_AUTOCONF_GLIBC_PREREQ(maj, min) 0
+#endif
+
+/* BUILD2_AUTOCONF_FREEBSD_PREREQ(maj, min)
+ *
+ * Return 1 if the given version number is >= the FreeBSD version, or 0
+ * otherwise.
+ */
+#if defined(__FreeBSD__) && defined(__FreeBSD_version)
+#  define BUILD2_AUTOCONF_FREEBSD_PREREQ(maj, min) \
+     (__FreeBSD_version >= (maj)*100000 + (min)*1000)
+#else
+#  define BUILD2_AUTOCONF_FREEBSD_PREREQ(maj, min) 0
+#endif
+
+/* BUILD2_AUTOCONF_OPENBSD_PREREQ(yyyymm)
+ *
+ * Return 1 if the given release date is >= the release date of the OpenBSD
+ * version, or 0 otherwise.
+ */
+#if defined(__OpenBSD__) && defined(OpenBSD)
+#  define BUILD2_AUTOCONF_OPENBSD_PREREQ(yyyymm) (OpenBSD >= (yyyymm))
+#else
+#  define BUILD2_AUTOCONF_OPENBSD_PREREQ(yyyymm) 0
+#endif
+
+/* BUILD2_AUTOCONF_NETBSD_PREREQ(maj, min)
+ *
+ * Return 1 if the given version number is >= the NetBSD version, or 0
+ * otherwise.
+ */
+#if defined(__NetBSD__) && defined(__NetBSD_Version__)
+#  define BUILD2_AUTOCONF_NETBSD_PREREQ(maj, min) \
+     (__NetBSD_Version__ >= (maj)*100000000 + (min)*1000000)
+#else
+#  define BUILD2_AUTOCONF_NETBSD_PREREQ(maj, min) 0
+#endif
+
+/* BUILD2_AUTOCONF_MACOS_PREREQ(maj, min)
+ *
+ * Return 1 if the given version number is >= the Mac OS version, or 0
+ * otherwise.
+ */
+#if defined(__APPLE__) && defined(__MACH__)
+#  if !defined(__MAC_OS_X_VERSION_MIN_REQUIRED)
+#    error __MAC_OS_X_VERSION_MIN_REQUIRED not defined
+#  endif
+#  if __MAC_OS_X_VERSION_MIN_REQUIRED <= 1090
+#    define BUILD2_AUTOCONF_MACOS_PREREQ(maj, min) \
+       (__MAC_OS_X_VERSION_MIN_REQUIRED >= (maj)*100 + (min)*10)
+#  else
+#    define BUILD2_AUTOCONF_MACOS_PREREQ(maj, min) \
+       (__MAC_OS_X_VERSION_MIN_REQUIRED >= (maj)*10000 + (min)*100)
+#  endif
+#else
+#  define BUILD2_AUTOCONF_MACOS_PREREQ(maj, min) 0
+#endif